This invention relates to a system of heating the service or operator's cabin of a machine, such as a mobil crane, or the like, or a motor vehicle equipped with an internal combustion engine comprising a hydraulic pump, a heat exchanger located in the service cabin or in the hot-air supply line thereof, a heat generator connected between the pump and the heat exchanger in the oil circuit of the internal combustion engine haing suction and return oil lines respectively extending from the pump and from the heat exchanger to an oil sump of the engine, an oil line by-passing the heat generator and being controlled by a directional control valve, and an oil line by-passing the heat exchanger and being controlled by a control mechanism.
In German Application No. P 29 49 834.8, a related system of the aforementioned type is disclosed in which the heat generator comprises a variable throttle or nozzle, the temperature being regulated by means of a control mechanism in the oil delivery pipe behind the high-pressure lift pump, and by-pass lines for the pressure-relief (nozzle) and heat-transfer devices are controlled, inter alia, by thermostatic means, so that only one by-pass or pressurization is provided for these two devices. Such an arrangement, however, achieves no optimum results either from a standpoint of adjustability of from a utilization of the heat supplied by the internal combustion engine. The gradual shutoff of a partial flow, for the purpose of regulating the temperature development, functions to control at the same time the mass flow and the pressure level upstream of the pressure-relief element. Such control is over-sensitive and reacts to each variation in the quantity of oil delivered with a change in the heat reduction. As a pressure-relief element, the nozzle provided is extremely unsuitable for such an arrangement in which the lift pump supplies a volume flow which varies greatly with engine speed. A large nozzle orifice size permits no temperature development when the flow rate is small, while a small nozzle orifice size does not allow the throughput delivered by the pump at high speeds and leads to heavy losses in the pump. While the temperature control is generally effected by the control mechanism or behind the high-pressure lift pump and, thus, directly affects the temperature development, the shutdown of the heater with a simultaneous heating of the heating medium can only be effect upstream of the heat exchanger. This requires two control mechanisms for controlling the heating device which inevitably leads to two control elements and, because of the large number of combination positions, results in a complex control system with mutual interference between the control mechanisms.